As a projection type image display apparatus for displaying an image by projecting light onto a screen, various types of apparatuses have been proposed, including an apparatus commercially available, which is a so-called “optical projector.” A basic principle of such a projection type image display apparatus is to produce a two-dimensional original image by utilizing a spatial light modulator such as a liquid crystal microdisplay or a DMD (Digital Micromirror Device), and magnifies and projects the two-dimensional original image on a screen by utilizing a projection optical system.
A general optical projector adopts a system which illuminates a spatial light modulator such as a liquid crystal display using a white light source such as a high pressure mercury lamp, magnifies a modulated image thus obtained, by means of lenses and projects the same on a screen. For example, Japanese Unexamined Patent Publication No. 2004-264512 discloses a technology that divides white light generated by an ultrahigh pressure mercury lamp into three primary color components of R, G, and B by means of a dichroic mirror, guides these lights to spatial light modulators for each primary color, synthesizes modulated images thus produced for the primary colors by means of a cross dichroic prism and projects a synthesized image on a screen.
However, the service life of a high brightness discharge lamp such as a high pressure mercury lamp is comparatively short, wherein if such a lamp is utilized in an optical projector, etc., the lamp needs to be frequently replaced. Further, since it is necessary to utilize a comparatively large optical system such as a dichroic mirror to extract lights of the primary color components, there is a disadvantage that the entire apparatus becomes large-sized. Therefore, a system using a coherent light source such as a laser has been proposed. For example, a semiconductor laser, which is widely utilized in industry, has a remarkably long service life in comparison with a high brightness discharge lamp such as a high pressure mercury lamp. Also, since the semiconductor laser is a light source capable of producing light of a single wavelength, a spectroscopic instrument such as a dichroic mirror is no longer required, and there is an advantage that the entire apparatus can be made small-sized.
On the other hand, in a system using a coherent light source such as a laser, a new problem of generating speckles is brought about. The speckles form a spot-like pattern appearing when coherent light such as a laser light is irradiated on a diffusing surface, and are observed as spot-like unevenness in brightness when they appear on a screen, and become a factor that physiologically causes harmful effects to an observer.
For example, when one point on a screen is indicated with a laser pointer, the spot of the laser light appears as a bright glare on the screen. This is caused by the occurrence of speckle noise on the screen. It is considered that the reason why speckles are caused when coherent light is used is that coherent lights reflected at portions of the diffusing and reflecting surface of a screen, etc., interfere with each other due to extremely high coherency. For example, a detailed theoretical consideration on occurrence of speckles is described in “Speckle Phenomena in Optics, Joseph W. Goodman, Roberts & Co., 2006.”
In use as a laser pointer or the like, a small spot is only seen by an observer, so that a severe problem does not occur. However, when a laser light source is used as an image display apparatus, an image must be displayed on the entire screen having a wide region, so that if speckles occur on the screen, they give physiologically harmful effect to an observer to cause symptoms such as feeling sick.
Of course, several detailed methods for reducing the above-described speckle noise have been proposed. For example, Japanese Unexamined Patent Publication No. H06-208089 discloses a technology by which laser light is irradiated onto a scatter plate, scattered light obtained from the scatter plate is guided to a spatial light modulator and the scatter plate is driven to rotate by a motor, whereby reducing speckles. Also, Japanese Unexamined Patent Publication No. 2004-144936 discloses a technology for reducing speckles by oscillating a scatter plate disposed between a laser light source and a spatial light modulator.
However, to rotate or oscillate the scatter plate, a large-scale mechanical drive mechanism is required, so that the apparatus is entirely increased in size, and power consumption is also increased. With this method, light from the laser light source is scattered by the scatter plate, so that a part of the laser light does not contribute to image display at all and is wasted. Further, even when the scatter plate is rotated or oscillated, the position of the optical axis of the illumination light does not change, so that speckles occurring on the diffusing surface of the screen cannot be sufficiently reduced.
Therefore, an object of the present invention is to provide a technology for efficiently and sufficiently reducing the occurrence of speckles in a projection type image display apparatus using a coherent light source.